You can stay competitive in a horizontal line of business by capturing more of the market share. One way to do that is to merge with another business that sells similar products.
Vertical integration is when you operate all aspects of the supply chain. This sometimes leads to cost savings and more efficiency. In the case of cannabis, that would mean you have some combination of the following businesses: grow business, transport business, dispensary, delivery business, testing business, and manufacturer business. You should only vertically integrate if you can save on costs by operating in different parts of the industry.
A horizontal line of business would be any one of the business activities in the supply chain.